The capacity of visual working memory for features and conjunctions

Short-term memory storage can be divided into separate subsystems for verbal information and visual information, and recent studies have begun to delineate the neural substrates of these working-memory systems. Although the verbal storage system has been well characterized, the storage capacity of visual working memory has not yet been established for simple, suprathreshold features or for conjunctions of features. Here we demonstrate that it is possible to retain information about only four colours or orientations in visual working memory at one time. However, it is also possible to retain both the colour and the orientation of four objects, indicating that visual working memory stores integrated objects rather than individual features. Indeed, objects defined by a conjunction of four features can be retained in working memory just as well as single-feature objects, allowing sixteen individual features to be retained when distributed across four objects. Thus, the capacity of visual working memory must be understood in terms of integrated objects rather than individual features, which places significant constraints on cognitive and neurobiological models of the temporary storage of visual information.     

Storage of features, conjunctions, and objects in visual working memory.

Working memory can be divided into separate subsystems for verbal and visual information. Although the verbal system has been well characterized, the storage capacity of visual working memory has not yet been established for simple features or for conjunctions of features. The authors demonstrate that it is possible to retain information about only 3–4 colors or orientations in visual working memory at one time. Observers are also able to retain both the color and the orientation of 3–4 objects, indicating that visual working memory stores integrated objects rather than individual features. Indeed, objects defined by a conjunction of four features can be retained in working memory just as well as single-feature objects, allowing many individual features to be retained when distributed across a small number of objects. Thus, the capacity of visual working memory must be understood in terms of integrated objects rather than individual features. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Symbolic control of visual attention: The role of working memory and attentional control settings.

This study examined how 1 symbol is selected to control the allocation of attention when several symbols appear in the visual field. In Experiments 1-3, the critical target feature was color, and it was found that uninformative central arrows that matched the color of the target were selected and produced unintentional shifts of attention (i.e., involuntary, initiated slowly, producing long-lasting facilitatory effects). Experiment 4 tested whether such selection is the result of an attentional filter or of a competition bias due to a match of incoming information against integrated object representations stored in working memory. Here, the critical feature was shape and color was irrelevant, but matching color arrows were still selected. Thus, features of objects in working memory will bias the selection of symbols in the visual field, and such selected symbols are capable of producing unintentional shifts of attention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feature-based memory-driven attentional capture: Visual working memory content affects visual attention.

In 7 experiments, the authors explored whether visual attention (the ability to select relevant visual information) and visual working memory (the ability to retain relevant visual information) share the same content representations. The presence of singleton distractors interfered more strongly with a visual search task when it was accompanied by an additional memory task. Singleton distractors interfered even more when they were identical or related to the object held in memory, but only when it was difficult to verbalize the memory content. Furthermore, this content-specific interaction occurred for features that were relevant to the memory task but not for irrelevant features of the same object or for once-remembered objects that could be forgotten. Finally, memory-related distractors attracted more eye movements but did not result in longer fixations. The results demonstrate memory-driven attentional capture on the basis of content-specific representations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Infant visual recognition memory: Independent contributions of speed and attention.

Relations between infant visual recognition memory and later cognition have fueled interest in identifying the underlying cognitive components of this important infant ability. The present large-scale study examined three promising factors in this regard--processing speed, short-term memory capacity, and attention. Two of these factors, attention and processing speed (but, surprisingly, not short-term memory capacity), were related to visual recognition memory: Infants who showed better attention (shorter looks and more shifts) and faster processing had better recognition memory. The contributions of attention and processing speed were independent of one another and were similar at all ages studied--5, 7, and 12 months. Taken together, attention and speed accounted for 6%-9% of the variance in visual recognition memory, leaving a considerable, but not unexpected, portion of the variance unexplained. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Visual working memory for observed actions.

Human society depends on the ability to remember the actions of other individuals, which is information that must be stored in a temporary buffer to guide behavior after actions have been observed. To date, however, the storage capacity, contents, and architecture of working memory for observed actions are unknown. In this article, the author shows that it is possible to retain information about only 2-3 actions in visual working memory at once. However, it is also possible to retain 9 properties distributed across 3 actions almost as well as 3 properties distributed across 3 actions, showing that working memory stores integrated action representations rather than individual properties. Finally, the author shows that working memory for observed actions is independent from working memory for object and spatial information. These results provide evidence for a previously undocumented system in working memory for storing information about actions. Further, this system operates by the same storage principles as visual working memory for object information. Thus, working memory consists of a series of distinct yet computationally similar mechanisms for retaining different types of visual information. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of visual working memory in attentive tracking of unique objects.

When tracking moving objects in space humans usually attend to the objects’ spatial locations and update this information over time. To what extent do surface features assist attentive tracking? In this study we asked participants to track identical or uniquely colored objects. Tracking was enhanced when objects were unique in color. The benefit was greater when the distance between distractors and targets was smaller, but was eliminated when the objects changed colors 1 to 4 times per second, even though at any instant they were always uniquely colored. In addition, tracking uniquely colored objects impaired a secondary color-memory task more than tracking identical objects, and holding several colors in working memory eliminated the advantage of tracking uniquely colored objects. Contrary to previous studies showing that feature information is poorly retained during tracking, these findings indicate that surface properties are stored in visual working memory to facilitate tracking performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A central capacity limit to the simultaneous storage of visual and auditory arrays in working memory.

If working memory is limited by central capacity (e.g., the focus of attention; N. Cowan, 2001), then storage limits for information in a single modality should apply also to the simultaneous storage of information from different modalities. The authors investigated this by combining a visual-array comparison task with a novel auditory-array comparison task in 5 experiments. Participants were to remember only the visual, only the auditory (unimodal memory conditions), or both arrays (bimodal memory conditions). Experiments 1 and 2 showed significant dual-task tradeoffs for visual but not for auditory capacity. In Experiments 3-5, the authors eliminated modality-specific memory by using postperceptual masks. Dual-task costs occurred for both modalities, and the number of auditory and visual items remembered together was no more than the higher of the unimodal capacities (visual: 3-4 items). The findings suggest a central capacity supplemented by modality- or code-specific storage and point to avenues for further research on the role of processing in central storage. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Binding objects to locations: The relationship between object files and visual working memory.

The relationship between object files and visual working memory (VWM) was investigated in a new paradigm combining features of traditional VWM experiments (color change detection) and object-file experiments (memory for the properties of moving objects). Object-file theory was found to account for a key component of object-position binding in VWM: With motion, color memory came to be associated with the new locations of objects. However, robust binding to the original locations was found despite clear evidence that the objects had moved. This latter binding appears to constitute a scene-based component in VWM, which codes object location relative to the abstract spatial configuration of the display and is largely insensitive to the dynamic properties of objects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Familiarity enhances visual working memory for faces.

Although it is intuitive that familiarity with complex visual objects should aid their preservation in visual working memory (WM), empirical evidence for this is lacking. This study used a conventional change-detection procedure to assess visual WM for unfamiliar and famous faces in healthy adults. Across experiments, faces were upright or inverted and a low- or high-load concurrent verbal WM task was administered to suppress contribution from verbal WM. Even with a high verbal memory load, visual WM performance was significantly better and capacity estimated as significantly greater for famous versus unfamiliar faces. Face inversion abolished this effect. Thus, neither strategic, explicit support from verbal WM nor low-level feature processing easily accounts for the observed benefit of high familiarity for visual WM. These results demonstrate that storage of items in visual WM can be enhanced if robust visual representations of them already exist in long-term memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A visual short-term memory advantage for objects of expertise.

Visual short-term memory (VSTM) is limited, especially for complex objects. Its capacity, however, is greater for faces than for other objects; this advantage may stem from the holistic nature of face processing. If the holistic processing explains this advantage, object expertise--which also relies on holistic processing--should endow experts with a VSTM advantage. The authors compared VSTM for cars among car experts and car novices. Car experts, but not car novices, demonstrated a VSTM advantage similar to that for faces; this advantage was orientation specific and was correlated with an individual's level of car expertise. Control experiments ruled out accounts based solely on verbal- or long-term memory representations. These findings suggest that the processing advantages afforded by visual expertise result in domain-specific increases in VSTM capacity, perhaps by allowing experts to maximize the use of an inherently limited VSTM system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of attention in the maintenance of feature bindings in visual short-term memory.

This study examined the role of attention in maintaining feature bindings in visual short-term memory. In a change-detection paradigm, participants attempted to detect changes in the colors and orientations of multiple objects; the changes consisted of new feature values in a feature-memory condition and changes in how existing feature values were combined in a binding-memory condition. In the critical experiment, a demanding visual search task requiring sequential shifts of spatial attention was interposed during the delay interval of the change-detection task. If attention is more important for the maintenance of feature bindings than for the maintenance of unbound feature values, the attention-requiring search task should specifically disrupt performance in the binding-memory task. Contrary to this proposal, it was found that memory for bindings and memory for features were equally impaired by the search task. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feature binding, attention and object perception

The seemingly effortless ability to perceive meaningful objects in an integrated scene actually depends on complex visual processes. The 'binding problem' concerns the way in which we select and integrate the separate features of objects in the correct combinations. Experiments suggest that attention plays a central role in solving this problem. Some neurological patients show a dramatic breakdown in the ability to see several objects; their deficits suggest a role for the parietal cortex in the binding process. However, indirect measures of priming and interference suggest that more information may be implicitly available than we can consciously access.     

Orienting attention in visual working memory reduces interference from memory probes.

Given a changing visual environment, and the limited capacity of visual working memory (VWM), the contents of VWM must be in constant flux. Using a change detection task, the authors show that VWM is subject to obligatory updating in the face of new information. Change detection performance is enhanced when the item that may change is retrospectively cued 1 s after memory encoding and 0.5 s before testing. The retro-cue benefit cannot be explained by memory decay or by a reduction in interference from other items held in VWM. Rather, orienting attention to a single memory item makes VWM more resistant to interference from the test probe. The authors conclude that the content of VWM is volatile unless it receives focused attention, and that the standard change detection task underestimates VWM capacity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Compression in visual working memory: Using statistical regularities to form more efficient memory representations.

The information that individuals can hold in working memory is quite limited, but researchers have typically studied this capacity using simple objects or letter strings with no associations between them. However, in the real world there are strong associations and regularities in the input. In an information theoretic sense, regularities introduce redundancies that make the input more compressible. The current study shows that observers can take advantage of these redundancies, enabling them to remember more items in working memory. In 2 experiments, covariance was introduced between colors in a display so that over trials some color pairs were more likely to appear than other color pairs. Observers remembered more items from these displays than from displays where the colors were paired randomly. The improved memory performance cannot be explained by simply guessing the high-probability color pair, suggesting that observers formed more efficient representations to remember more items. Further, as observers learned the regularities, their working memory performance improved in a way that is quantitatively predicted by a Bayesian learning model and optimal encoding scheme. These results suggest that the underlying capacity of the individuals’ working memory is unchanged, but the information they have to remember can be encoded in a more compressed fashion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Trade-offs between gaze and working memory use.

Eye movements during natural tasks suggest that observers do not use working memory to capacity but instead use eye movements to acquire relevant information immediately before needed. Results here however, show that this strategy is sensitive to memory load and to observers' expectations about what information will be relevant. Depending upon the predictability of what object features would be needed in a brick sorting task, subjects spontaneously modulated the order in which they sampled and stored visual information using working memory more when the task was predictable and reverting to a just-in-time strategy when the task was unpredictable and the memory load was higher. This self organization was evidenced by subjects' sequence of eye movements and also their sorting decisions following missed feature changes. These results reveal that attentional selection, fixations, and use of working memory reflect a dynamic optimization with respect to a set of constraints, such as task predictablity and memory load. They also reveal that change blindness depends critically on the local task context, by virtue of its influence on the information selected for storage in working memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Limitations of object-based feature encoding in visual short-term memory.

The present study investigated object-based feature encoding in visual short-term memory for 2 features within the same dimension that occur on different parts of an object. Using the change-detection paradigm, this experiment studied objects with 2 colors and objects with 2 orientations. Participants found it easier to monitor 1 rather than both features of such objects, even when decision noise was properly controlled for. However, no object-based benefit was observed for encoding the 2 features of each object that were of the same dimension. When similar stimuli were used but the 2 features of each object were from different dimensions (color and orientation), an object-based benefit was observed. These results thus impose a major constraint on object-based feature encoding theories by showing that only features from different dimensions can benefit from object-based encoding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Constructing Visual Representations of Natural Scenes: The Roles of Short- and Long-Term Visual Memory.

A "follow-the-dot" method was used to investigate the visual memory systems supporting accumulation of object information in natural scenes. Participants fixated a series of objects in each scene, following a dot cue from object to object. Memory for the visual form of a target object was then tested. Object memory was consistently superior for the two most recently fixated objects, a recency advantage indicating a visual short-term memory component to scene representation. In addition, objects examined earlier were remembered at rates well above chance, with no evidence of further forgetting when 10 objects intervened between target examination and test and only modest forgetting with 402 intervening objects. This robust prerecency performance indicates a visual long-term memory component to scene representation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Life-span development of visual working memory: When is feature binding difficult?

We asked whether the ability to keep in working memory the binding between a visual object and its spatial location changes with development across the life span more than memory for item information. Paired arrays of colored squares were identical or differed in the color of one square, and in the latter case, the changed color was unique on that trial (item change) or was duplicated elsewhere in the array (color-location binding change). Children (8-10 and 11-12 years old) and older adults (65-85 years old) showed deficits relative to young adults. These were only partly simulated by dividing attention in young adults. The older adults had an additional deficiency, specifically in binding information, which was evident only when item- and binding-change trials were mixed together. In that situation, the older adults often overlooked the more subtle, binding-type changes. Some working memory processes related to binding undergo life-span development in an inverted-U shape, whereas other, bias- and salience-related processes that influence the use of binding information seem to develop monotonically. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Understanding the function of visual short-term memory: Transsaccadic memory, object correspondence, and gaze correction.

Visual short-term memory (VSTM) has received intensive study over the past decade, with research focused on VSTM capacity and representational format. Yet, the function of VSTM in human cognition is not well understood. Here, the authors demonstrate that VSTM plays an important role in the control of saccadic eye movements. Intelligent human behavior depends on directing the eyes to goal-relevant objects in the world, yet saccades are very often inaccurate and require correction. The authors hypothesized that VSTM is used to remember the features of the current saccade target so that it can be rapidly reacquired after an errant saccade, a task faced by the visual system thousands of times each day. In 4 experiments, memory-based gaze correction was accurate, fast, automatic, and largely unconscious. In addition, a concurrent VSTM load interfered with memory-based gaze correction, but a verbal short-term memory load did not. These findings demonstrate that VSTM plays a direct role in a fundamentally important aspect of visually guided behavior, and they suggest the existence of previously unknown links between VSTM representations and the occulomotor system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)